EP-A-1 273 640 describes 2K [2-component] coating compositions composed of a polyol component and a crosslinker component composed of aliphatic and/or cycloaliphatic polyisocyanates, with 0.1 to 95 mol % of the originally free isocyanate groups present having undergone reaction with bisalkoxylsilylamine. These coating compositions can be used for OEM, production-line finishing, and feature good scratch resistance in conjunction with high resistance to environmental effects. However, these coating compositions have a particularly severe postcrosslinking propensity, since the conversion on thermal curing after application is inadequate. The consequences of this for the weathering stability in particular are negative.
DE-A-100 60 327 describes 1K [1-component] coating compositions comprising, as crosslinkers, blocked isocyanates in which 1 to 90 mol % of the originally free isocyanate groups present have undergone reaction with at least one secondary 3-aminopropyl-trialkoxysilane. These modified blocked isocyanates enhance the scratch resistance of the resultant coatings. Moreover, the modified blocked isocyanates exhibit a reduced crystallization propensity and solubility improved relative to the corresponding unmodified isocyanates.
WO 08/074,491 describes thermally curable coating compositions based on aprotic solvents and comprising polyols and polyisocyanates in which some of the isocyanate groups have undergone reaction with a mixture of a bisalkoxysilylamine (Ia), preferably bis[3-(trimethoxysilyl)propyl]amine, and a monoalkoxy-silylamine (IIa), preferably N-[3-(trimethoxysilyl)-propyl]butylamine. The coating compositions lead to highly scratch-resistant and acid-resistant coatings, and coatings and paint systems, especially clearcoats, can be produced even in film thicknesses >40 μm without stress cracks appearing.
These coating compositions known from EP-A-1 273 640, DE-A-100 60 327 and WO 08/074,491 may comprise typical rheological assistants. Further details as to how the problem of the sagging propensity of the coating, which occurs at relatively high film thicknesses, can be influenced, however, are absent from the publications. Indeed, as the film thickness of a coating goes up, there is an increase in the sagging propensity F, which correlates proportionally with the third power of the film thickness and with the viscosityF˜d3/ηwhere d is the film thickness of the coating and η is the viscosity of the coating composition.
In order, therefore, to reduce the sagging propensity for a given film thickness, it is necessary to increase the viscosity of the coating composition. Within the industry, on the other hand, there is a requirement to achieve the desired, target film thicknesses with as few application procedures as possible, this being synonymous with an increased film thickness per application step. In the area of the automobile industry and its supplier industry, in particular, the coating compositions for the color and/or effect coatings are applied by means of spraying, and this means that the viscosity of the coating compositions at the outlet nozzle ought to be relatively low.
This required viscosity behavior on the part of the coating materials is typically brought about by adding rheological assistants to the coating compositions. Compounds which have proven to be particularly appropriate in this context include various urea derivatives, obtained, for example, by reacting 1,6-hexamethylene diisocyanate with benzylamine or with methoxypropylamine. Rheological assistants of such kinds are described in DE-A-27 51 761, in EP-A-192 304 and in EP-A-198 519, for example.
One ongoing development of these rheological assistants takes the form of urea derivatives obtained by reacting 1,6-hexamethylene diisocyanate with chiral amino acids, as is described in WO 05/005558. These urea derivatives result in improved surface quality in the resultant coatings.
Furthermore, WO02/098943 discloses coating compositions which comprise a urea-based rheological assistant obtainable by reacting a diisocyanate with a mixture of a primary and/or secondary diamine, such as ethylenediamine, for example, and a primary and/or secondary monoamine, such as n-hexylamine or methoxypropylamine, for example. The coatings obtained from these coating compositions are notable for improved optical properties. The use of amines containing silane groups for preparing the rheological assistants, however, is not described in the specification.
Finally, WO 06/131314 describes silane-modified urea derivatives and their use in silane-crosslinking inks, varnishes and coatings. These silane-modified urea derivatives are obtained by stoichiometric reaction of diisocyanates with primary or secondary aminosilanes, preferably by stoichiometric reaction of diisocyanates with primary aminosilanes.
These urea derivatives used as rheological assistants are typically precipitated in a binder or a vehicle, such as a polyacrylate resin or a polyester resin, and this material is then added to the coating composition or, in the case of 2-component coating materials, to the millbase. These rheological assistants therefore supply the coating composition with further binder, and in certain circumstances this may lead to problems, such as instances of incompatibility. Alternatively it would be necessary to prepare and stock a large number of different rheological assistants each with binders adapted to the particular coating compositions.
Furthermore, in coating compositions which are baked at low temperatures of not more than 90° C., the only urea derivatives which can be used are those referred to as transparent urea derivatives, since the opaque types lead to paint film clouding. Finally, more particularly, the more innovative rheological assistants, resulting in improved properties, are decidedly expensive products.
The problem addressed by the present invention was therefore that of providing coating compositions which ought to have an improved sagging propensity even without the use of known rheological assistants, such as those based on urea derivatives, i.e., which ought to be able to be applied even at relatively high film thicknesses >40 μm, more particularly >50 μm, without runs with a length of >=1 cm occurring. The coating compositions ought to lead, even when cured at low temperatures of not more than 90° C., of the kind customary in the area of the coating of plastics parts, as for example for components for installation in or on automobiles, or in the area of automotive refinish, to coatings with a very good optical quality and with high transparency.
Furthermore, the coating compositions, depending on intended use, ought to lead to highly scratch-resistant and acid-resistant coatings. Moreover, the coatings and paint systems, especially the clearcoats, ought to be able to be produced even in film thicknesses >40 μm without stress cracks appearing. This is an essential prerequisite for the use of the coatings and paint systems, more particularly the clearcoats, in the technologically and esthetically particularly demanding sector of automotive OEM finishing. Over and above these requirements, the new coating compositions are to be producible simply and with very good reproducibility, and are not to give rise to environmental problems in the course of their application.